There’s danger in the water, and it involves ornamental fish. Dirty aquariums can lead to killer fish, says a new study. Although, it’s likely that most ornamental fish are already livid and violent. How can you tell that a fish is mad? In the study, mild-mannered fish flared their fins, while ticked-off fish killed each other. Ronald Oldfield, a biology instructor at Case Western Reserve University, thinks it may show insight into human action. "This study might help us to better understand how human behavior changes when people are placed in different social environments," he said.